1. Field of the Invention
The present invention relates to an X-ray exposure mask to be used in fabrication processes of integrated circuits and fabrication method thereof.
2. Description of the Prior Art
In recent years, X-ray exposure method has been widely used in lithograph processing in order to increase the degree of integration in integrated circuits. Generally, X-ray masks used for X-ray exposure are fabricated by forming X-ray absorption patterns composed of an absorption layer absorbing X-rays formed on a membrane through which X-rays easily transmit. In 1:1 proximity X-ray exposure using the above mentioned X-ray mask, the thickness of the X-ray absorption layer becomes an important factor because mask contrast, which is defined by the ratio of the X-ray transmittance at the part without absorption materials to the X-ray transmittance at the part composed of absorption materials, is mainly determined by the thickness of the absorption layer. Mask contrast is given by 1/exp(-.mu.t) where t is the thickness of the absorption layer measured by nm, .mu. is the linear absorption coefficient with respect to the irradiated X-ray measured by 1/nm. Conventionally, as it is assumed that the higher the mask contrast, the better the exposure performance which includes the resolution of X-ray exposure, the resolution of transferred patterns and the exposure dose margin, the thickness of the absorption layer has been taken to be as large as possible, so long as the absorption patterns can be replicated. In this case, the value of mask contrast between 7 to 10 is adopted for a criteria whether the mask may be used or not. For example, in case that tantalum (Ta) is used for a material of the absorption layer, where the thickness of the absorption layer is 0.65 .mu.m, the mask contrast is about 7 for the synchrotron radiation with the peak wavelength of 0.8 nm. Hence, conventional masks have been fabricated so that the thickness of absorption layers may be not less than 0.65 .mu.m in order to make the mask contrast at least 7. This criteria for determining the thickness of absorption layers assumes that the plane size of patterns to be replicated is as large as 1 .mu.m. In case of transferring fine patterns including less than 1 .mu.m, what should be considered is an effective change of the exposure contrast due to the X-ray diffraction and the mutual interference.
The X-ray after passing through the membrane gives rise to diffraction. In addition, the X-ray passing through the absorption layer suffers the decrease of the intensity and the phase shift, and furthermore, the X-ray after passing through the absorption layer interferes with diffracted X-rays. The range where diffraction and interference occur is dependent upon the wavelength of the X-ray and the proximity gap, that is, the distance between the mask and the sample. The range where diffraction and interference occurs most ofter is limited to the area closest to pattern edges. Therefore, the smaller the pattern size, the larger the effect of diffraction and interference which leads to changes in the transmitted X-ray intensity distribution.
In case diffraction and interference occur, the effective exposure contrast can not be defined simply by the ratio of the X-ray transmittance at the location without absorption materials to the X-ray transmittance at the location composed of absorption materials. Alternately, in this case, the effective contrast should be estimated on the basis of the X-ray intensity reaching to the sample substrate facing the absorption layer pattern. In some cases where diffraction and interference occur, as described in detail later, it may be found that the X-ray intensity distribution on the region on the sample corresponding to the membrane is not uniform but has a minimum value and that the X-ray intensity distribution on the region of the sample corresponding to the X-ray absorption pattern contains a maximum peak. In this case, the effective exposure contrast can be formulated by the ratio of the minimum value of the X-ray intensity distribution on the region not corresponding to the absorber pattern to the maximum value of the X-ray intensity distribution on the region corresponding to the absorber pattern.
The effect of the X-ray diffraction and the mutual interference is described in published papers, M. Weiss et al., Microelectric Eng., 6, pp. 265-271, 1987 and Y. C. Ku et al., J. Vac. Sci. & Technol., B6, pp. 150-153, 1988. In these papers, the exposure resolution depends upon the phase shift and the intensity change of X-ray through the absorption layers.
In the following, the effect of the X-ray diffraction and the mutual interference over the effective exposure contrast is described.
In FIG. 1, a cross-sectional view of a conventional X-ray exposure mask is shown. The mask comprises an X-ray transmission layer (membrane) 1 with a thickness of 2 .mu.m composed of silicon nitride and an X-ray absorption layer or X-ray absorber 2 with a thickness of Da=0.65 .mu.m composed of tantalum (Ta) formed on the X-ray transmission layer 1. The X-ray absorber 2 is so patterned as to have an open window 3 with its width W1 being 0.1 .mu.m. Since this mask has a thick X-ray absorber 2, if the width of the open window 3 is large enough and greater than 1 .mu.m, the effective exposure contrast (in this case, it is defined by the ratio of the maximum X-ray exposure intensity on the region of the sample which corresponds to the region on the X-ray transmission layer 1 where the X-ray absorber 2 does not exist to the maximum X-ray exposure intensity on the region of the sample which corresponds to the region where the X-ray absorber 2 exists), and the exposure dose margin (it determines the range on the sample where the X-ray exposure patterns can be replicated in responsive to patterns of the X-ray absorber 2) on the sample can be established to be relatively high, and the size of transferred patterns can be well controlled. However, in the case where the width of the window is as small as 0.1 .mu.m, there may be problems such as the X-ray diffraction and the X-ray mutual interference.
In FIGS. 2A and 2B, the X-ray exposure intensity distribution on the surface of the sample in irradiating the X-ray having the peak wavelength of 0.8 nm is shown. As for FIG. 2A, the distance between the X-ray mask and the surface of the sample, that is , the proximity gap G, is 30 .mu.m. As for FIG. 2B the proximity gap G is 20 .mu.m. In FIGS. 2A and 2B, the horizontal axis represents the distance measured from a point on the sample surface corresponding to the center of the window 3 along the direction of the width of the window, and the vertical axis represents the relative value of the X-ray exposure intensity. The X-ray after passing through the window 3 diffracts and the X-ray after passing through the X-ray absorber layer 2 gives rise to phase shift, and finally, the diffracted X-rays from the window 3 and the X-rays transmitted through the X-ray absorber 2 interfere with each other. As a result, the X-ray exposure intensity distribution on the sample is influenced considerably, and as shown in FIGS. 2A and 2B, the effective exposure contrast and the exposure dose margin are very low. Specifically, in the case where the proximity gap G is 30 .mu.m, as shown in FIG. 2A, the X-ray exposure intensity at the sample positions corresponding to an inner area under the X-ray absorber 2 is higher than the X-ray exposure intensity at the sample position corresponding to the window 3. This means that the pattern defined by the X-ray absorber 2 can not be transferred completely onto the sample. This situation does not change whether the shape of the window 3 is a well-like with a right square form or a long extended groove form.
In FIG. 3, a cross-sectional view of another conventional X-ray exposure mask is shown. The structure of the mask shown in FIG. 3 is almost similar to that shown in FIG. 1 except that the thickness Db of the X-ray absorber 2' is 0.3 .mu.m. In FIGS. 4A and 4B, the X-ray exposure intensity distribution on the surface of the sample in irradiating the X-ray having the peak wavelength of 0.8 nm is shown. As for FIG. 4A, the distance between the X-ray mask shown in FIG. 3 and the surface of the sample, that is, the proximity gap G, is 30 .mu.m. As for FIG. 4B the proximity gap G is 20 .mu.m. Similarly as in FIGS. 2A and 2B, the effect of the mutual interference of the diffracted X-ray and the transmitted X-ray is shown. In this case, the effective exposure contrast and the exposure dose margin are higher than those in the case of the mask shown in FIG. 1, but the amount of X-rays transmitted through the X-ray absorber also increases, and therefore the mask contrast is as low as from 2 to 3. The mask shown in FIG. 3 has a defect such as a fog resulting from a leakage of irradiated X-ray inside the masked part.
In FIG. 5, a cross-sectional view of a conventional X-ray mask having lines-and-spaces patterns is shown. On the 2 .mu.m-thick X-ray transmission layer (membrane) 1 composed of silicon nitride, a 0.65 .mu.m-thick tantalum film is formed as the X-ray absorber 4. The X-ray absorber 4 is composed of repetitive patterns of absorber 4A where the width W3 of the absorber is 0.1 .mu.m and the distance W2 between the adjacent absorber is 0.1 .mu.m. In other words, the windows 5 having a 0.1 .mu.m-width are spaced at a distance of 0.1 .mu.m. Reference numeral 4B is an X-ray absorber for defining the windows at both sides, the width of which is generally greater than that of the absorber 4A.
In FIG. 6, the X-ray exposure intensity distribution on the surface of the sample in irradiating the X-ray having the peak wavelength of 0.8 nm is shown, where the mask shown in FIG. 5 is placed on the sample with the proximity gap G of 20 .mu.m. Due to the X-ray diffraction and the phase shift, the mask shown in FIG. 5 has such a defect which not only lowers the effective exposure contrast and the exposure dose margin but also is incapable of replicating the patterns exactly on the sample substrate in response to patterns defined by the X-ray absorber 4.
FIG. 7 is a cross-sectional view of the mask with the thickness Db of the X-ray absorption layer being 0.3 .mu.m and with the other configuration features similar to those of the mask shown in FIG. 5. Under the same conditions as obtained in FIG. 6, in irradiating X-rays, the X-ray exposure intensity distribution on the sample is shown in FIG. 8. The X-ray intensity distribution shown in FIG. 8 shows better correspondence with the patterns given by the mask patterns than those in FIG. 5. Both the effective exposure contrast and the exposure dose margin are increased. However, the amount of transmitted X-rays is too large at the region on the sample where the outermost X-ray absorbers 4B exist, because the mask contrast is low. When using the mask shown in FIG. 8, the fog may occur at the periphery of the mask pattern.
FIG. 9 is a cross-sectional view of the X-ray mask having an isolated pattern. In FIG. 9, the X-ray mask is composed of a stripe-like or a square form X-ray absorber 6 of tantalum having its own width W4 and thickness Da, where Da is 0.65 .mu.m and W4 is 0.2 .mu.m, formed on the 2 .mu.m-thick X-ray transmission layer 1 made of silicon nitride. FIG. 10 shows the X-ray exposure intensity distribution on the sample in irradiating X-rays having peak wavelength of 0.8 nm and placing the mask on the sample with proximity gap G of 20 .mu.m. As the width W4 of the X-ray absorber 6 is as small as 0.2 .mu.m, not only the exposure contrast and the exposure dose margin are relatively low, but also there are still problems such as being incapable of replicating the transferred patterns exactly on the sample substrate in response to patterns defined by the X-ray absorber 6. This is because the X-ray exposure intensity at the center of the X-ray absorber 6 measured in the horizontal direction is relatively larger than that measured at another positions.
FIG. 11 is a cross-sectional view of the mask with the thickness Db of the X-ray absorber 6' being 0.3 .mu.m and with the other configuration features similar to those of the mask shown in FIG. 9. Under the same conditions as obtained in FIG. 10, in irradiating X-rays, the X-ray exposure intensity distribution on the sample is shown in FIG. 12. The X-ray exposure intensity at the center of the X-ray absorber 6' measured in the width direction is lower than that in FIG. 10. However, the X-ray exposure intensity at the region on the sample corresponding to the region where the X-ray absorber 6' exists is relatively higher than that in the case of the mask shown in FIG. 9, and therefore, the fog may occur at the region on the sample where the X-ray absorber 6' exists. This is a defect of the mask defined in FIG. 11.
In FIG. 13, a cross-sectional view of another conventional X-ray exposure mask is shown. The mask having X-ray absorbers formed on the X-ray transmission layer 1 composed of 2 .mu.m-thick silicon nitride has patterned regions A, B and C, respectively, in the same manner as patterns shown in FIGS. 1, 5 and 9. In the region A, the width W1 of the window 3 defined by the X-ray absorbers 2 is 0.1 .mu.m. In the region B, the width W2 of the window 5 defined between the adjacent X-ray absorbers 4A and 4B and the width W3 of the X-ray absorbers 4A is 0.1 .mu.m. In the region C, the width W4 of the isolated X-ray absorber 6 is 0.1 .mu.m. All of the absorbers 2, 4A, 4B and 6 are composed of tantalum films and their thickness Da is 0.65 .mu.m. The X-ray exposure intensity distribution in the region A corresponding to 30 .mu.m or 20 .mu.m of the proximity gap is similar to that given by FIG. 2A or 2B. When the gap G is 20 .mu.m, the X-ray exposure intensity distributions in the region B and the region C are similar to those given by FIGS. 6 and 10, respectively.
FIG. 14 shows a cross-sectional view of the mask with the thickness Db of the X-ray absorbers 2', 4'A, 4'B and 6' being 0.3 .mu.m and with the other configuration features similar to those of the mask shown in FIG. 13. The patterns of the X-ray absorbers formed in the regions A' B' and C' are similar to those shown in FIGS. 3, 7 and 11, respectively. In using the mask shown in FIG. 14, the X-ray exposure intensity distributions on the sample at the region A, B and C are similar to those shown in FIGS. 4A, 4B, 8 and 12, respectively, under the same conditions as these figures.
For resolving a degradation of the resolution due to aforementioned diffraction of the light and its mutual interference, in optical lithography technologies, a phase-shift mask is proposed. The following references refer to this technology; Marc D. Levenson et al., IEEE Trans. ED., 29, pp. 1828-1836, 1982, Marc D. Levenson et al., IEEE Trans. ED., 31, pp. 753-763, 1984, Mark D. Prouty et al., SPIE, 470, pp 228-232, 1984, Japanese Patent Application Publication No. 62-50811 (1987), Japanese Patent Application Laying-open No. 58-173744(1983), 61-292643(1986), 62-67514(1987), and 1-147458(1989).
These papers disclose that a phase shifter is composed of materials enabling only a shift of the phase of the incident wave by 180.degree. without reducing the intensity of the incident wave and that the phase shifter is placed at open aperture of the masking members in order to improve the exposure resolution. However, as the material used for the phase shifter is different from that used for the masking members, the fabrication process for forming the phase shifter may be more complicated and testing the fabricated mask may be difficult, either of which leads to practical problems. Phase shifter may be valid specifically for masks including regularly repetitive patterns, and hence, its applicable patterns are limited.
On the other hand, the following references disclose the technology for improving the resolution of the X-ray exposure by using the phase shifting effect; Japanese Patent Application Laying-open No. 2-52416 (1990), Shinya Hasegawa et al., Microelectronic Eng., 9, pp. 127-130, 1989, and Yoshiki Yamakoshi et al., Appl.Optics., 25, pp. 928-932, 1986. Hasegawa et al. propose that by tapering the side wall of the absorption layer patterns to cause the phase shift at the pattern edge, the X-ray intensity distribution may be improved. Yamakoshi et al. propose an X-ray mask on which a phase shifter is formed for shifting the phase by 180.degree. by the analogy of the phase shifting mask used in the optical lithography. However, in the conventional X-ray mask using the phase shifting effect, since only one kind of phase shifter can be used regardless of the kinds and sizes of the patterns of the X-ray absorber, the. X-ray intensity distribution is improved under a limited condition of the kind and size of the X-ray absorber and the proximity gap.
As described above, in exposing X-rays by using conventional X-ray masks with higher contrast, even if the proximity gap (distance between the mask and the wafer) is greater than or equal to 30 .mu.m, the mask containing only patterns with a size greater than 0.3 .mu.m can be exactly replicated. However, in the case of using the mask containing patterns with a size less than 0.3 .mu.m, the exposure resolution is greatly affected by the X-ray diffraction and interference, and specifically as for the patterns with plane size being less than 0.2 .mu.m, pattern replication can not be performed easily. Hence, in order to replicate patterns less than 0.2 .mu.m by using conventional X-ray masks, it has been necessary to reduce the proximity gap to about 10 .mu.m, which is not practical. And furthermore, there is a limited number of combinations of patterns of the X-ray absorber and the value of the proximity gap which will attain an optimal X-ray exposure intensity distribution.
And furthermore, conventional methods for fabricating X-ray absorber patterns are categorized into two classes: dry etching technology and plating technology. In either technology, in case of increasing the thickness of the absorption layer, as the aspect ratio (the ratio of the height of the pattern to the width of the pattern) increases, it is very difficult to fabricate patterns exactly and precisely, which leads to deterioration of quality of fabricated absorber patterns estimated by sizes and shapes. Specifically, if the size of patterns is less than 0.25 .mu.m, the problem is that the degree of deterioration of fabricated patterns is extremely high.